Field of the Invention
The technology disclosed in the present specification relates to a bootstrap compensation circuit and a power module, in particular to a bootstrap compensation circuit which supplies current to a bootstrap circuit used to control a totem-pole connected high-side switching element, and relates to a power module including the bootstrap compensation circuit.
Description of the Background Art
Generally, in the case of a power device control circuit for driving each of totem-pole connected power devices such as an inverter circuit, an insulated individual power source needs to be used for each of a control circuit on a high side and a control circuit on a low side.
A power device control circuit for controlling a single-phase inverter needs to have totally three insulated power sources, that is, two insulated power sources on the high side and one insulated power source on the low side.
Further, a power device control circuit for controlling a three-phase inverter needs to have three insulated power sources on the high side, and hence needs to have totally four insulated power sources; thus, there is a problem that the size of the power device control circuit is increased.
On the other hand, in order to reduce the number of power sources, there is also a configuration in which bootstrap circuits are used as the power sources for the control circuits on the high side so that the number of power sources of the power device control circuit for controlling the three-phase inverter is reduced to one.
However, a bootstrap circuit cannot charge a capacitor when a power device on a high side is operating in an on-state. Therefore, when the power device on the high side operates in an on-state for a long time, a power source voltage goes down; therefore, it is difficult to apply a bootstrap circuit to a control method in which the power device operates in an on-state for a long time.
To address this issue, a bootstrap compensation circuit is proposed (see, for example, Japanese Patent Application Laid-Open No. 2011-234430) in order to make it possible to sufficiently charge a capacitor in a bootstrap circuit and to simplify and downsize the circuit.
However, in the case that a bootstrap compensation circuit as described in Japanese Patent Application Laid-Open No. 2011-234430 is used, the response speed is an issue to be addressed when taking into consideration a possible application of the bootstrap compensation circuit in which a high voltage such as 600 V or 1200 V is applied.
A resistive voltage divider circuit in a bootstrap compensation circuit needs to be provided between a high-voltage side of a power source voltage and a reference potential. Further, a resistive voltage divider circuit needs to have a resistor with a high resistance value in order to reduce current flowing through voltage dividing resistors.
As a result, a current consumption in the voltage dividing resistors is reduced, but at the same time a time constant of a CR circuit formed by the voltage dividing resistors and the accompanying parasitic capacitance is increased, whereby the response speed is lowered.
The output of the output circuit is switched from an on-state to an off-state due to the switching of the output of the resistive voltage divider circuit when the power source voltage becomes higher than the reference potential. However, because the output response of the resistive voltage divider circuit is slower than the change of the power source voltage, the switching of the output circuit between the on-state and the off-state is also delayed.
In the case that a load circuit of the output circuit is provided between the output circuit and the reference potential, an excessive electric power is applied to the load circuit and the output circuit since the power source voltage rises and until the output circuit goes into the off-state.
Alternatively, in order to address the above-described decrease in the response speed, there is a method in which a transient response signal is used to achieve a high-speed response. However, even in this case, the output circuit goes into the on-state after disappearance of the transient response signal which appears quickly but does not last long and until generation of a direct current signal which lasts long but is generated late, and an excessive electric power is also applied to the load circuit and the output circuit.
Note that an allowable power consumption is normally several hundreds of milliwatts or lower, considering a heat dissipation property of an IC package. In order to satisfy the above-described condition when a high voltage such as 600 V or 1200 V is applied, it is necessary to use voltage dividing resistors with resistances of meg-ohms.
If the parasitic capacitance accompanying the above-described voltage dividing resistors is several picofarads, the time constant is calculated by a formula (MΩ×several picofarads), and the response speed is in the order of a microsecond. On the other hand, the transient response between the power source voltage on the high-voltage side and the reference potential is in the order of several kilovolts per microsecond. As a result, before the response occurs in a period in the order of a microsecond and the output circuit goes into the off-state, an excessive electric power such as several tens of watts is applied to the load circuit and the output circuit.